This invention relates to a process for improving the dielectric properties of solid tantalum capacitors; and more particularly to improving the storage life of hermetically-sealed solid electrolyte tantalum capacitors which are stored at elevated temperatures without any applied voltage.
It is well known in the prior art that a solid electrolyte tantalum capacitor may be manufactured by the following method. A sintered porous tantalum body is anodized in any of a variety of electrolytes so as to form a film of tantalum pentoxide (the dielectric) over all exposed surfaces of the pellet. Several coatings, typically eight to ten, of a manganous nitrate solution are applied over the dielectric, including regions in the pores of the tantalum body. Each coating is fired at a temperature in the range of 250.degree. C. to 400.degree. C. so as to convert the manganous nitrate to the semiconducting coating of manganese dioxide (MnO.sub.2). The composite coating of the MnO.sub.2 is then coated with a layer of graphite from an aqueous suspension, and a layer of silver from a paint suspension. The unit at this stage is encapsulated by any of several methods, typically by being hermetically sealed in a metal can or by encapsulation in a plastic coating.
Hermetically sealed solid electrolyte tantalum capacitors can undergo some degree of dielectric degradation when stored at elevated temperatures in the absence of an applied bias. This degradation is manifested after the storage period by a high leakage current upon the application of voltage. The leakage current is found to increase with time as the capacitors are maintained at elevated temperatures. Typically a capacitor with a leakage current of less than 0.1 microampere before storage may show a leakage of 1 to 10 microamperes after several days of storage at 125.degree. C. The problem is primarily associated with capacitors rated at the higher voltages, i.e. at 35V and higher.
It is postulated that the increase in leakage current is due to faults occurring in the oxide caused by stress imposed by elevated temperatures. One possible picture is that the oxide cracks, with slight shearing taking place, exposing unanodized tantalum which contacts MnO.sub.2 resulting in a high leakage current site.
Though this degradation of the dielectric oxide can occur, it is possible to treat the oxide by thermal and electrochemical techniques such that the stability at elevated temperature storage is dramatically improved. It has been shown by Smyth, Shirn and Tripp in J. Electrochem. Soc., Vol. 110, p. 1264, 1963, that exposure of anodized tantalum to high temperature, greater than 200.degree. C., modifies the dielectric oxide film by causing oxygen to migrate from the oxide into the adjacent tantalum thereby yielding an oxide with increased conductivity due to vacancies in the oxide. During the manufacture of a solid electrolyte tantalum capacitor the tantalum/Ta.sub.2 O.sub.5 structure is exposed to temperatures in excess of 200.degree. C. repeatedly in order to thermally create the manganese dioxide solid electrolyte.
The deterioration to the oxide increases (1) with increasing pyrolysis temperature used in the manufacturing process and (2) with increasing phosphate and glycol concentration in the anodization electrolyte. The ultimate effect on the capacitor is experienced in increased sensitivity of the capacitance to temperature, frequency and bias.
Methods that have been employed in the prior art to overcome these problems, which deal with the A.C. parameters of the capacitors, have been revealed in GB No. 1,082,390 published Sept. 6, 1967 and in U.S. Pat. No. 3,653,119 issued Apr. 4, 1972.
It has now been observed that the thermal degradation of the dielectric oxide during manufacture of the capacitor also can cause instability of the D.C. leakeage current on elevated temperature storage.
The commonly accepted manufacturing method for solid tantalum capacitors involves anodization or oxide growth in electrolytes composed of phosphate, normally from phosphoric acid, ethylene glycol and water.
The solid electrolyte is deposited by thermal decomposition of manganous nitrate at temperatures greater than 250.degree. C.
The combination of the electrolytes and the elevated temperature of pyrolysis cause deterioration of the oxide film which subsequently results in unstable leakage current on elevated temperature shelf test.